Ensemble forecasting is a promising tool to aid in making informed decisions against risks of coastal storm surges. Although tropical cyclone (TC) ensemble forecasts are commonly used in operational numerical weather prediction systems, their potential for disaster prediction has not been maximized. Here we present a novel, efficient, and practical method to utilize a large ensemble forecast of 1000 members to analyze storm surge scenarios toward effective decision making such as evacuation planning and issuing surge warnings. We perform the simulation of TC Hagibis (2019) using the Japan Meteorological Agency’s (JMA) non-hydrostatic model. The simulated atmospheric predictions were utilized as inputs for a statistical surge model named the Storm Surge Hazard Potential Index (SSHPI) to estimate peak surge heights along the central coast of Japan. We show that Pareto optimized solutions from an ensemble storm surge forecast can describe potential worst (maximum) and optimum (minimum) storm surge scenarios while exemplifying a diversity of trade-off surge outcomes among different coastal places. For example, some of the Pareto optimized solutions that illustrate worst surge scenarios for inner bay locations are not necessarily accountable for bringing severe surge cases in open coasts. We further emphasize that an in-depth evaluation of Pareto optimal solutions can shed light on how meteorological variables such as track, intensity, and size of TCs influence the worst and optimum surge scenarios, which is not clearly quantified in current multi-scenario assessment methods such as those used by JMA/National Hurricane Center in the United States.

In this study, 42 years of tidal records and landfall TC best tracks in Japan were used to demonstrate that TC pre-landfall forward speed is significantly correlated with maximum storm surge height. Coastal morphology was the determining factor for the correlation between storm surge and TC forward speed. Fast-moving TCs tended to amplify the storm surge along open coastlines (Pearson correlation coefficient, R = 0.62), but reduce it in semi-enclosed bays (R = -0.52). The negative correlation contrasts with the general perception that the coincidence of TC wind speed and forward speed vectors generates a larger storm surge. The influence of coastal morphology was most prominent for TCs with a central pressure lower than 956 hPa. Tropical cyclone (TC) operational forecasts are continuously improving; however, there is still scope to improve the precision of storm surge predictions. These findings could contribute to the improvement of storm surge forecasting and provide emergency management personnel with more precise early warnings of dangerous storm surges.

Over the past several decades, scientists have focused on numerical sensitivity analysis to explain the relative importance of tropical cyclone (TC) forward speed on storm surge and wave height predictions. These past studies performed numerical analyses, but the results have not been sufficiently compared with long term observations. In this study, 42 years of tidal records and landfall TC best tracks in Japan were analyzed, demonstrating that TC pre-landfall forward speed is significantly correlated with both maximum storm surge and significant wave height. Coastal horizontal morphology was the determining factor among these correlations. Fast-moving TCs tended to amplify the storm surge along open coastlines (Pearson correlation coefficient, R = 0.62), but reduce it in semi-enclosed bays (R = -0.52). A similar tendency has also been observed for the case of wave height (open coast, R = 0.62; bay, R = -0.52). The negative correlation contrasts with the general perception that the coincidence of TC wind speed and forward speed vectors generates a larger storm surge. The influence of coastal morphology was most prominent for TCs with a central pressure lower than 956 hPa. Tropical cyclone operational forecasts are continuously improving; however, there is still scope to improve the precision of storm surge - wave predictions. These findings may be beneficial in two main areas. Firstly, considering TC transitional speed and coastal geometry (open coastline or bays) - meteorologists and oceanographers could provide more comprehensive surge-wave forecasts, and emergency management personnel could use pre-landfall forward speed for more precise early warning. Secondly, coastal areas at risk with no access to advanced weather forecasting could use these empirical findings along with other TC intensity related information (e.g., wind speed, central pressure, radius) for improvement of early-warning activities.

Over the past several decades, operational forecasts of typhoon tracks have improved steadily. However, storm surge forecast skills have experienced rather modest improvements as it has been assumed to be primarily a function of maximum typhoon wind speed. In this study, numerical sensitivity experiments have been conducted for the semi-enclosed Tokyo Bay to investigate the existence of any connection between typhoon size and peak storm surge height. The radius of the maximum wind (Rmax) derived based on the 50-kt wind radius (R50) is used to define the size of a typhoon. The results show that peak storm surge height tends to increase as the size of typhoon becomes larger, which may also be supported by historical observations. Storm size plays a significant role in surge generation, particularly for very large typhoons making landfall in the upper bay. Analyses show that for a given hypothetical typhoon, the water level in the inner bay is increased by 1 m, changing Rmax from 13 km to 89 km. The findings of this study will be beneficial for the storm surge modeling community as it gives insight into the role of typhoon size, which is essential to forecast peak surge height precisely.